1. Field of the Invention
The present invention relates to a document reading apparatus capable of selecting a reading mode for the image of a document placed on a document table and read by a line sensor moving relatively to the document, or a reading mode for the image of a document fed relatively to a fixed line sensor and read by the line sensor.
2. Description of the Related Art
FIG. 4 schematically shows the arrangement of a conventional reading unit serving as part of a digital copier. The reading unit includes a transparent document table 2 for supporting a document D1 not to be moved for reading, a carriage 3 disposed below the document table 3, and a line sensor 1 mounted on the carriage 3. The carriage 3 is movable in a sub-scanning direction perpendicular to a main scanning direction, and the line sensor 1 is mounted parallel to the main scanning direction. In the table-document reading mode, the line sensor 1 reads an image from the document D1 on the document table 2 while moving in the sub-scanning direction together with the carriage 3. The reading unit further includes a document tray 5 for receiving a stack of documents D2 to be moved for reading, a feed roller 4 for feeding each of the documents D2 from the document tray 5, and a discharge tray 6 for receiving the document D2 discharged by the feed roller 4. In the feeder-document reading mode, the line sensor 1 is set at a reference reading position and reads an image of the document D2 fed by the feed roller 4 and passing through the sensor 1.
In such a configuration, the document D1 is placed on the document table 2 with its reading surface faced down, whereas the document D2 is placed on the document tray 5 with its reading surface faced up. For this reason, if the document D1 is placed with its top edge oriented to the far side of the document table 2 in FIG. 4, then the right edge of the reading surface is located at a position close to the line sensor 1 and the left edge thereof is located at a position remote from the line sensor 1. In contrast, if the document D2 is placed with its top edge oriented to the far side of the document tray 5 in FIG. 4, then the left edge of the reading surface is located at a position close to the line sensor 1 and the right edge thereof is located at a position remote from the line sensor 1. That is, reading of an image starts from the right edge of the reading surface in the table-document reading mode and from the left edge of the reading surface in the feeder-document reading mode.
Such a difference in the reading start edge causes a problem in the process of enlarging or reducing the size of an image read from part of the document. To be more specific, assume that the copy magnification is set to 200% to print an image of the area located on the upper right-hand side of the document shown in (a) of FIG. 5 on a sheet having the same size as the document. When the document is placed on the document table 2 as a document D1 not moved for reading, the image of the area located on the upper right-hand side of the document D1 is converted into image data to be printed as shown in (b) of FIG. 5. On the other hand, if the document is placed on the document tray 5 as a document D2 moved for reading, the image of the area located on the upper left-hand side of the document D2 is converted into image data to be printed as shown in (c) of FIG. 5.
To cope with the problem, correction of the read range is required in the feeder-document reading mode. However, this increases the burden on the user. Instead, an image of the document D2 may be read for the entire length in the sub-scanning direction to carry out an image processing for the area located on the upper right-hand side of the document D1. However, the processing time will be increased due to the image data read from an unnecessary part of the document D2.